Pressure
From Halbeeg, the open encyclopedia · Af-Soomaali
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Pressure is the measure of force applied perpendicularly to a free surface or area. It is defined as the force distributed over the surface area on which the force acts. When equal force is applied to a smaller surface, pressure increases; when the surface is larger, pressure decreases.
Pressure is a scalar quantity, meaning it has only magnitude and no direction, although force does have direction. The international unit (SI) is the pascal (Pa), equal to one newton per square metre. Other common units include the bar, the atmosphere, millimetres of mercury (mmHg), and pounds per square inch (psi).
Definition and calculation
The fundamental equation is p = F/A, where p is pressure, F is the force acting perpendicularly on the surface, and A is the surface area. If the force is not perpendicular to the surface, only the perpendicular component is taken into account.
Pressure in fluids
In stationary fluids, pressure increases with depth. The relationship is expressed as p = ρgh, where ρ is the density of the fluid, g is the gravitational acceleration, and h is the depth of the fluid column. For this reason, pressure on the ocean floor is extremely high. Pascal's principle states that pressure applied to an enclosed, incompressible fluid is transmitted uniformly in all directions; this is the basis of hydraulic systems in jacks and presses.
Atmospheric pressure and measurement
Atmospheric pressure results from the weight of air pressing on the Earth's surface and decreases with altitude. At sea level, atmospheric pressure is approximately 101,000 pascals (1 atm). A barometer measures atmospheric pressure, while manometers and other gauges measure gas and fluid pressure. A distinction exists between absolute pressure, measured from a vacuum, and gauge pressure, measured relative to atmospheric pressure.
Applications
Pressure is used in the study of chemistry, mechanical engineering, air conditioning, and medicine—for example, in measuring blood pressure, expressed in mmHg. The behaviour of gases, as described by the ideal gas law (pV = nRT), relates pressure, volume, temperature, and the number of particles.